BE Computer Engineering (IOE, TU) Basic Electrical Engineering (IOE, EE 451) Question Paper 2079

(a) State and explain Thevenin's theorem. Outline the step-by-step procedure for obtaining the Thevenin equivalent of a linear network containing both independent and dependent sources. [6]

(b) For the network shown below, a 24 V source is connected across terminals A-B through a 4 Ω resistor, with a 6 Ω resistor in series feeding node C, and a 12 Ω resistor from C to ground. Determine the Thevenin equivalent (V_Th and R_Th) seen by a load resistor R_L connected between node C and ground, and hence find the value of R_L for maximum power transfer and the corresponding maximum power delivered to it. [6]

(a) Derive the EMF equation of a single-phase transformer and define its transformation ratio. [5]

(b) A 25 kVA, 2200/220 V, 50 Hz single-phase transformer gave the following test results: Open-circuit test (LV side): 220 V, 1.5 A, 90 W; Short-circuit test (HV side): 75 V, 11.4 A, 280 W. Draw the approximate equivalent circuit referred to the HV side and determine the efficiency and percentage voltage regulation at full load, 0.8 power factor lagging. [7]

(a) With a neat sketch, explain the working principle and construction of a DC shunt generator. Derive the expression for the generated EMF. [6]

(b) A 220 V DC shunt motor draws a line current of 40 A on full load. The armature resistance is 0.25 Ω and the field resistance is 110 Ω. If the rated speed is 1200 rpm, determine the back EMF on full load and the speed at which the motor will run when the load is reduced such that the armature current falls to 20 A (assume flux constant). [6]

(a) Define resonance in a series RLC circuit. Derive expressions for the resonant frequency, quality factor (Q) and bandwidth, and sketch the variation of current with frequency. [6]

(b) A series RLC circuit has R = 10 Ω, L = 0.1 H and C = 100 μF, connected across a 230 V variable-frequency supply. Calculate (i) the resonant frequency, (ii) the impedance at resonance, (iii) the current at resonance, and (iv) the quality factor and bandwidth of the circuit. [6]

A balanced three-phase star-connected load of impedance (8 + j6) Ω per phase is connected to a 400 V, 50 Hz, three-phase supply. Determine the phase current, line current, power factor, and the total active and reactive power drawn by the load.

An iron ring of mean circumference 60 cm and cross-sectional area 5 cm² has an air gap of 2 mm cut in it. The ring is wound with 500 turns. Taking the relative permeability of iron as 800, calculate the current required to produce a flux of 0.5 mWb in the air gap. Neglect leakage and fringing.

State the superposition theorem and explain its limitations. Discuss why it cannot be applied directly to calculate power dissipated in a resistor.

Compare the construction and operating principle of a Permanent Magnet Moving Coil (PMMC) instrument with a Moving Iron (MI) instrument. State, with reasons, why a PMMC instrument cannot be used directly for AC measurement and how its range is extended for measuring large currents.

A 3-phase, 4-pole, 50 Hz induction motor runs at 1440 rpm on full load. Calculate the synchronous speed, the slip, and the frequency of the rotor EMF. Briefly explain why an induction motor can never run at synchronous speed.

(a) Define average value, RMS value and form factor of an alternating quantity, and derive the form factor of a sinusoidal voltage. [3]

(b) Explain the concept of power factor and discuss two practical methods used to improve the power factor of an industrial load. [3]

(a) Explain the difference between a two-winding transformer and an auto-transformer, and state the principal advantages and one limitation of an auto-transformer. [3]

(b) Distinguish between hysteresis loss and eddy current loss in a transformer core, and state how each is minimized in practice. [3]